app: make GimpBucketFillTool a GimpColorTool.

In particular, it allows to easily color pick. This just makes sense as
the bucket fill is definitely what one could call a "color tool", and
being able to easily change color without having to constantly switch to
color picker tool nor open a color chooser dialog is a must.
The fill type option (FG/BG/Pattern) was already mapped to the common
toggle behavior key (Ctrl on Linux), which is commonly used for
switching to color picker on paint tools. So I decided to remap the fill
type switch to GDK_MOD1_MASK (Alt on Linux) to keep consistent with
other tools (at the price of a change for anyone used to this modifier,
though I doubt it was that much used).
I also made possible to combine the 2 modifiers (so you could pick the
foreground or background color with ctrl and ctrl-alt).